Non-Destructive Mirror of a Multires Modifier Sculpt
Use Animation Nodes add-on, it is open-source and totally free. I'd suggest a setup like this:

The node tree duplicates A then modify its scale (new y = y*(-1)). In your case, you may want to do it with x instead. I tried to sculpt A.  After I mouse-released the sculpt brush, B would be updated too. In this approach, I can't sculpt the new object (so-called B) directly.


This issue as well as a few similar ones will be solved by Everything Nodes project for vanilla Blender but the implementation is not done yet and there is still a long way to go.

Here are some of Chris-P's Animation Nodes Youtube tutorials. I have no affiliation with the author. However, the first few episodes of the series were very useful to me as beginner. Feel free to comment about better tutorial please.